1. Field of the Invention
The invention relates to a method for operation of a braking system of a vehicle that can be driven in a hybrid form. The invention also relates to a braking system and to a vehicle.
2. Description of the Related Art
A hybrid drive has at least two different energy converters and two different energy stores. The converters typically are an internal combustion engine and at least one electric motor. The energy store for the internal combustion engine is a supply of combustible fuel. The energy store for the electric motor may be a battery that can be charged, a high-performance capacitor or a flywheel.
A hybrid vehicle has an advantage over conventional vehicles with an internal combustion engine in that the hybrid vehicle can recover a large proportion of the braking energy, except for an efficiency loss. This feedback or temporary storage of the energy, is referred to as recuperation and takes place during active braking of the vehicle and during overrunning. Overrunning identifies the condition when the vehicle is being propelled forwards only by its own inertia mass and the internal combustion engine is not actively driving the vehicle. The recovered or recuperated braking energy is stored temporarily in the energy store, and can be used again when required. In this case, the electric motor is operated as a generator and acts as a mechanical/electrical converter. However, the generator operation of the electrical machine also results in braking. Thus, in addition to the conventional mechanical brakes, which are generally operated hydraulically, pneumatically or electromechanically, the electrical machine that can be operated as a generator represents a so-called regenerative brake or electrical brake.
Braking can be carried out entirely using the conventional brakes. In this case, the electrical machine is not activated and there is no recuperation. In addition, it is also feasible for a braking process to be carried out additionally or exclusively by the electrical machine being operated in the generator mode. In principle, it is desirable to make use of as high a regenerative braking force as possible when the motor vehicle is being decelerated to recover as much energy as possible by recuperation during deceleration.
The option of recuperative braking problematically affects the brake pressure that must be provided for the conventional mechanical brake. A reduced brake pressure or an increased brake pressure is required depending on whether braking is carried out with or without recuperation. Stronger brake pedal operation therefore is required for a braking process without recuperation than in the case for a braking process with recuperation. This is particularly dangerous when driving a hybrid-drive vehicle in extreme driving conditions, as is in the field of racing sports where the vehicle occasionally is operated in the limit area. However, this assumes that the brake pedal can predetermine a braking process exactly and reproducibly. Different brake pedal pressures are present for the same braking process depending on whether there is recuperation, and hence the vehicle driver cannot be sure that a specific brake pedal pressure will always result in the same braking.
The object of the present invention is therefore to provide better braking operation for a hybrid vehicle, particularly in racing conditions.